Musar על שבת 109:7

Our sages, of blessed memory, spoke strongly regarding one who is able to rebuke someone but refrains from doing so. They decreed his judgment to be that he himself will be held accountable for the sin of the sinners. In the Midrash, [they expounded: "'her princes were like deer [that find no pasture...]' (Eicha 1:6) - just like during a heat wave, these deer turn their faces one beneath the other, so too the great sages of Israel would see sin committed and turn their faces away from it. The Holy One, blessed be He, said to them: 'a time will come when I will do the same to them'" (Eicha Raba 1:13).